Variable speed photographic process printers that utilize CRT's for media exposure typically use linear deflection amplifiers to sweep the electron beam across the face of the CRT. A deflection cycle normally consists of a sweep, retrace, and hold period. During the sweep period, the CRT is unblanked and the beam is driven horizontally at a given vertical position while being modulated to expose the media. During the first half of the retrace period, the CRT is blanked and the beam is returned to a home position to wait for the start of the next sweep. Thus, the sweep and the first half of the retrace periods are typically fixed time intervals. The beam is then held in the home position for the hold period until a new sweep is initiated, therefore, the hold period is a variable time interval dependent on the print speed. When the next sweep is initiated, the second half of retrace is completed, sweeping the trace to one side of the CRT, and the forward sweep then follows immediately.
Prior art system have provided a means to produce a non-linear CRT yoke drive current in order to cause a linear beam sweep in the forward direction. This is necessary because of the inherent deflection and CRT geometric non-linearities, and amplifier slew limitations due to the inductive load presented by the yoke coil. To produce the non-linear current, prior art systems use a translation table, usually implemented in a read-only-memory, to translate a linear horizontal position counter into the non-linear current. This translation table provides control of the position of the electron beam, since a different current may be output for each position of the beam as it sweeps across the face of the CRT.
Prior art systems use the same translation table for both the forward and retrace sweeps, therefore the non-linearity for the forward sweep is mirrored in the retrace. The retrace profile thus influences the smoothness of beam motion during the forward sweep immediately following the retrace causing detectable non-linearities. The fact that the sweep and retrace profiles are inter-related complicates the objective of accurate linear beam velocity on the face of the CRT.
It is thus apparent that there is a need in the art for an improved system that uses independent sweep and retrace motion profiles to improve the beam behavior in the early period of the forward sweep following retrace. The independent profiles allow the retrace to be used to improve and enhance forward sweep motion after the beam turn around. The present invention meets this and other needs.
This application is related to U.S. Pat. No. 5,084,656 issued to Sturm, et al. Jan. 28, 1992, entitled "CRT Beam Deflection Control System", which is specifically incorporated herein by reference for all that is disclosed and taught therein.